Electrooptic devices for display using liquid crystals, electroluminescences or the like are actively applied to products exploiting such features as low power consumption and thin type, as one of flat panel displays which are alternatives to CRTs.
A liquid crystal display, for example, usually comprises a liquid crystal panel in which a liquid crystal layer is held between a TFT array substrate and a counter substrate having a color filter, and a polarizing plate provided in the liquid crystal panel, and further comprises a back light unit and a light source which are provided outside one of the substrates. Light from the light source is emitted by using the back light unit to the polarizing plate and the liquid crystal panel and passes therethrough, to thereby achieve an image of color display. An electroluminescence display device comprises a light emitting film using electroluminescence and a counter electrode which are provided on a TFT array substrate, and achieves display light by passing a current through the light emitting film.
On the TFT array substrate, a TFT and a pixel electrode are arranged in matrix. A drain electrode of the TFT and the pixel electrode are isolated from each other by an interlayer insulating film and joined to each other through a contact hole formed in the interlayer insulating film.
As a material of the pixel electrode, usually, a transparent conductive material such as indium oxide, zinc oxide or the like is used. In such a case, if aluminum or an aluminum alloy is used as a material of the drain electrode, an oxide layer is formed at a junction with the transparent conductive material and this increases the contact resistance at the junction. For this reason, as the material of the drain electrode, a refractory metal having a reducing power, such as chromium or titanium, is used in most cases.
As display devices using liquid crystal, electroluminescence or the like are substituted for other display devices and the scope of application thereof increases, there has been an increase in the requirements for upsizing, higher definition and higher intensity of screens. In order to maintain excellent image quality even in a large area screen, it is necessary to suppress an increase in the wire resistance for the wire length as the screen is upsized. When the wire width is made larger in order to reduce the wire resistance, however, the light transmission region (that is, numerical aperture) decreases and this causes a decrease in the luminance. For this reason, it is necessary to use a wire material of low resistance, such as aluminum or an aluminum alloy.
In the case where aluminum or an aluminum alloy is used for the drain electrode (i.e., a wire), the contact resistance increases at the junction with the pixel electrode (i.e., a transparent electrode). In order to solve this problem, proposed is a method in which a conductive layer other than aluminum is formed on a surface of the drain electrode (see, e.g., Patent Document 1). In this method, however, another film formation or patterning is needed to form the conductive layer and this disadvantageously increases the cost.
Another method, other than the above, is proposed, in which the aluminum or the aluminum alloy forming the upmost layer of the drain electrode, which is exposed from the contact hole, is removed by wet etching when the contact hole is formed in the interlayer insulating film (see, e.g., Patent Document 2). When isotropic etching such as wet etching is used, however, side etching proceeds and an inner wall of the contact hole thereby forms an overhanging shape (i.e., an eaves structure), not being in contact with the drain electrode. Therefore, the pixel electrode and the drain electrode are not sufficiently joined to each other and this may cause a conduction failure.
Also in the above another method, another film formation of one layer or patterning is needed, as well as allying of aluminum, to form the conductive layer and this disadvantageously increases the cost.
Further, when the contact hole is formed on the drain electrode (the wire), since the aluminum or the aluminum alloy forming the upmost layer of the drain electrode, which is exposed from the contact hole, is removed (hereinafter, this method is referred to as an “aluminum-removed contact structure”), the electric resistance of the wire thereby increases and this causes a delay in operating speed and degradation in reliability. In order to avoid this, input/output terminals for connection to array peripheral and external circuits and wiring patterns with retrofitted LSI chips are subject to some constraints (see, e.g., circuit diagrams shown in Patent Document 3).
Still another method is proposed, in which impurities are contained in the entire surface layer of the drain electrode (the wire) made of aluminum or an aluminum alloy, to thereby suppress formation of the oxide layer at the junction with the pixel electrode (the transparent electrode) (see, e.g., Patent Document 4, and hereinafter, this method is referred to as a “surface-layer impurity containing aluminum structure”). This method does not cause an increase in the cost, nor does not cause an increase in the electric resistance or any conduction failure.
Further, in order to give constraint on power consumption and increase visibility in bright places, there are some cases where not only a back light but also extraneous light is used as the light source of the liquid crystal display. This is a system in which a reflecting electrode made of a material, such as aluminum or silver, having high reflectance in a visible light region as well as a transparent pixel electrode passing light therethrough is formed in an upper portion of the TFT, to reflect the extraneous light entering the liquid crystal panel from the outside and use the light as a display light (see, e.g., Patent Document 5, and hereinafter, this method is referred to as a “semitransparent liquid crystal display”).
Since the reflecting electrode is usually formed in a process step before or after the step of forming the pixel electrode, however, the step of forming the reflecting electrode needs to be added and this disadvantageously increases the cost.
Then, in the above surface-layer impurity containing aluminum structure, if the reflecting electrode is formed at the same time when the wire is formed of aluminum or an aluminum alloy, to thereby manufacture the above semitransparent liquid crystal display, there is no need to add another step of forming the reflecting electrode.
Since introduction of impurities into an aluminum surface layer reduces the reflectance of visible light, however, it is disadvantageously impossible to achieve a semitransparent liquid crystal display of high reflectance. With respect to an incident light having a wavelength of 550 nm, for example, the reflectance is 90% or more when the aluminum or the aluminum alloy is used, but if impurities are introduced, the reflectance decreases to 85% or less.    Patent Document 1: Japanese Patent Application Laid Open No. 4-253342 (page 3, lines 4 to 31, FIG. 1)    Patent Document 2: Japanese Patent Application Laid Open No. 9-244062 (page 5, right column, line 12 to page 7, left column, line 4, FIGS. 5 and 6)    Patent Document 3: Japanese Patent Application Laid Open No. 2006-107692 (FIGS. 19 and 20)    Patent Document 4: Japanese Patent Application Laid Open No. 11-284195 (FIG. 3)    Patent Document 5: Japanese Patent Application Laid Open No. 11-109417 (FIG. 1)